The present invention relates to a method and an apparatus for relieving a capacitor of a controlled series compensator as a function of the load on its arrester when high-current line faults occur.
In series compensation, capacitors are usually used in the line section in order to reduce the transmission angle, and the current-related voltage drop on the line, in stages. These are capacitor banks that are connected to and disconnected from the system either as a group or as a plurality of subcapacitors (segments) in series. The capacitor is connected and disconnected by opening or closing a parallel circuit breaker. Protection for the capacitor in the event of a network short circuit is provided by a parallel arrester, a triggerable spark gap, and/or the parallel circuit breaker.
A series compensation system may be used in which, by means of an inductor wired in parallel with the capacitor, the total impedance of the series compensator, by analogy with the TCR (thyristor controlled reactor) in a static compensator, is steplessly controlled with a power converter valve at high voltage by firing appropriately. A series compensator controlled in this manner is known as an ASC (advanced series compensator). With a series compensator controlled in this manner, the dynamics of series compensation can be improved and the total impedance is controllable within a certain range. Additionally, the impedance can be changed from capacitative to inductive.
Series compensators of this type are described in the article "Regulated Parallel and Series Compensators," printed in the German journal "Elektrie," volume 45, March 1991, pages 88-90. A series compensator of this type that is integrated into a transmission line is also described in the international patent document WO 87/04538.
One essential component in protecting the series capacitor of the controlled series compensator is non-linear resistors, also called arresters, for example MOVs (metal oxide varistors), which are connected electrically parallel to the series capacitor. These attesters are dimensioned so that at a given voltage they very quickly conduct current and thereby protect the series capacitor from longer-duration overload conditions.
For economic reasons, its energy absorption capacity is of course limited, and therefore a series compensation system also needs the ability to close a parallel bypass rapidly in order to protect the series capacitor and the arrester from overloads. This bypass has a circuit breaker or a spark gap and a damping circuit. The purpose of this damping circuit is to damp discharge effects that can occur in the event of line faults after the bypass closes.
In previous controlled series compensation systems for high-voltage electrical lines the series compensation systems needed to be shorted out, i.e. disconnected, in the event of high-current faults in order to provide protection against overload for the installed operating means needed for the purpose.
Each disconnection of the controlled series compensator reduces the availability, of the series compensation system and therefore the profitability of that system.